shower faucet slowing the water flow (w video)

Why is my shower faucet slowing the flow of the water after a few seconds?

I took a little 5 second video to show you what I mean - watch how it squeezes back in on it's own. Almost seems like it was designed to do that, but I can't imagine they would design the handle to actually give the customer a worse flow and crappy shower.

Some of the new Moen faucets will do that until they get a little dirty.
I haven't installed one like that in twenty years. The new ones are all pressure balanced. Is it a new "repair" meaning new cartridge replacement?
Do you you find an old non-code faucet at a big box store?

Thanks Terry for the reply, so what you're saying is that this will eventually go away after some use? I had a new moen kit (shower head, faucet, handles, trim ect) that I wanted to have put in, but the plumber who helped me said that would cost a bunch and require a lot of time opening the wall and he said he had a trim for that model in the van so all I'd need to do was put on a new handle. I'm not sure if he changed the cartridge. I can go back and look at the receipt, how much would a new cartridge cost roughly? The handle I got locally at the hardware store. Thanks again.